Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Various techniques have been developed for obtaining digital X-ray and gamma ray images of an object for purposes such as X-ray diagnostics, medical radiology, non-destructive testing, and so on. In one conventional approach, a flat-panel, two-dimensional, digital X-ray imager may include a plurality of active or passive detecting pixels formed on a silicon substrate. Each of such detecting pixels may consist of a photodiode and a plurality switches for resetting and selecting the photodiode. The active detecting pixel may further include an active element, such as a buffer amplifier of high input impedance and low output impedance. The use of active elements enables image production from very low light sources, but disadvantageously requires larger physical size and more complicated manufacturing processes.
In addition to structural differences, a passive X-ray imager is configured to measure the charges stored in the pixel directly, while an active X-ray imager is configured to measure the voltages across the pixel and calculate the charges stored in the pixel based on Coulomb's law (Q=CV). Since the photodiode and the switches also contribute to pixel capacitance, the measured pixel voltages may have a non-linear relationship with the amount of charges stored in the pixel during exposure in the active X-ray imager.
In a prior art imaging system, the pixel voltages may be reset by draining charges from the pixel through a semiconductor switch. The eventual level to which the pixel may reset to may vary due to thermal noises generated by the turn-on resistance of the semiconductor switch. The effect is called kTC noise and poses a fundamental limit to the sensitivity of the imaging system since the uncertainty of this reset level in the current readout period results in noises in the subsequent readout period. Therefore, there is a need for an imaging system which provides good low-signal sensitivity (particularly when adapted as a passive X-ray imager), good linearity (particularly when adapted as an active X-ray imager), and high sensitivity by reducing kTC noises.